1. The Field of the Invention
The present invention relates to breathable, waterproof sheet materials for protecting against water and other liquids, which may be used, for example, in fabricating waterproof clothing articles, and to methods for making such waterproof sheet materials.
2. The Prior Art
The evolution of protective rainwear has seen the production of garments made of so-called "oil skins" (cloth impregnated with oil to make it waterproof), cloth impregnated or covered with hydrophobic polymeric material such as polyurethane or polyvinyl chloride, and plastic sheet material such as polyvinyl chloride or polyethylene sheet materials. Such materials, however, for the most part do not allow for the evaporation of perspiration.
Fabrics treated with hydrophobic liquids or polymeric materials such as silicone or fluorocarbon oil or resin are materials known to provide adequate evaporation of perspiration, but they permit the passage of liquid water therethrough, commonly leaking when rubbed, touched, or otherwise contacted or flexed.
U.S. Pat. No. 4,194,041 Gore et al.) is representative of a number of patents which describe coatings or laminates purported to provide waterproof articles which do not leak when touched and are breathable. This patent describes a layered article for use in waterproof garments or tents comprising at least two layers; an interior, continuous hydrophilic layer that allows water to diffuse therethrough, prevents the transport of surface active agents and contaminating substances such as those found in perspiration, and is substantially resistant to pressure induced flow of liquid water, and a hydrophobic layer that permits the transmission of water vapor. The hydrophobic layer is preferably waterproof microporous polytetrafluoro-ethylene (PTFE) or polypropylene, which permits the passage of moisture vapor through the pores thereof. The hydrophilic layer transfers moisture vapor therethrough whereupon it passes through the porous hydrophobic layer. Various means of joining the layers are suggested including the application of hydraulic pressure to force the hydrophilic polymer to penetrate into the surface void spaces of the hydrophobic layer.
U.S. Pat. No. 4,443,511 (Worden et al.) discloses a layered article similar to that of U.S. Pat. No. 4,194,041, but formed of a breathable polytetrafluoroethylene first layer mechanically stretched at least about 5% beyond its yield point having a surface layer of elastomeric hydrophilic material having a surface in interlocking relationship with the inner face of the first layer. The interlocking relationship is said to be brought about by flowing the hydrophilic elastomeric material into the layer of hydrophobic material and then solidifying the hydrophilic material.
While these patents have alleviated some of the problems known to the art, they require lamination and the attendent deficiencies created thereby, e.g., an exposed weak hydrophilic layer subject to abrasion and separation and an exposed porous hydrophobic surface that is subject to contamination.
A significant advancement in the art which overcomes the aforementioned problems is disclosed in U.S. Pat. No. 4,613,544 (Burleigh). This patent describes waterproof, moisture vapor permeable, unitary, i.e., nonlaminated and single thickness, sheet materials that are ideally suited for use in waterproof garments, tents, and outdoor gear such as knapsacks and the like, combining exceptional durability with excellent hand or drape. These materials are made by taking stretched microporous films and coating them with a hydrophilic material or precursor thereof which solidifies and fills the pores of the microporous films.
The process disclosed in U.S. Pat. No. 4,613,544 provides good quality waterproof materials. However, the unitary sheet materials made in accordance with the process of that patent generally do not have sufficient dimensional stability or strength at the thicknesses typically used to be easily manufactured under the conditions of temperature or machine induced tension needed to transport the film and cure the hydrophilic material during manufacturing. This is partially because microporous films in general are somewhat thermally unstable, and thus shrink upon heating. Moreover, microporous films are extremely fragile, thereby rendering it very difficult to apply the hydrophilic material and fill the micropores of the microporous film without damaging the film. Hence, in the practice of the invention of U.S. Pat. No. 4,613,544, a supporting web or liner is often applied to the microporous film so as to stabilize it and protect it during processing. The hydrophilic material is then applied to the supported microporous film in accordance with U.S. Pat. No. 4,613,544, and if desired a layer of cloth fabric is applied thereto. Only then is the support web or liner removed.
As will be appreciated, the usage of such a support web or liner to support and protect the microporous film results in increased processing steps and decreased processing efficiency. Moreover, where such a support web or liner is employed, the process for forming the microporous film and the process of combining the hydrophilic material with the microporous film to produce the unitary waterproof material are conducted in separate processing steps using separate apparatus, again rendering the overall process less efficient than would be desired.
From the foregoing, it will be appreciated that it would be a significant advancement in the art to provide methods for preparing breathable, waterproof sheet materials similar to the type disclosed in U.S Pat. No. 4,613,544, without the need for a supporting web or liner, thereby greatly simplifying the process and allowing the microporous film to be formed, combined with the hydrophilic filler material, and laminated to a cloth fabric if desired, all in the same processing line. Such methods and waterproof materials obtained thereby are disclosed and claimed herein.